In view of high integration and high speed operation, an MRAM is a promising nonvolatile memory. In the MRAM, a magnetoresistive element showing a “magnetoresistance effect” such as the TMR (Tunnel MagnetoResistance) effect is used. In the magnetoresistive element, a Magnetic Tunnel Junction (MTJ) where, for example, a tunnel barrier layer is sandwiched by two ferromagnetic layers is formed. The two ferromagnetic layers include: a magnetization pinned layer (a pinned layer) whose direction of the magnetization is pinned; and a magnetization free layer (a free layer) whose direction of the magnetization can be switched.
It has been known that a resistance value (R+ΔR) of the MTJ of a case where the directions of magnetization of the pinned layer and the free layer are “antiparallel” becomes larger than a resistance value (R) of a case where the directions are “parallel” due to the magnetoresistance effect. The MRAM uses a magnetoresistive element having the MTJ as a memory cell, and stores data in a nonvolatile manner by using variation of the resistance value. For example, the antiparallel state is related to data “1”, and the parallel state is related to data “0”. Writing of data to a memory cell is carried out by switching the direction of magnetization of the free layer.
As a method for writing data to the MRAM, the “asteroid method” and the “toggle method” have been known. According to these writing methods, switching magnetization required to switch the magnetization of the free layer becomes large approximately in inverse proportion to a memory cell size. That is, the more the memory cell is refined, the more a writing current tends to increase.
As a writing method able to suppress the increase of the writing current caused by the refinement, a “spin transfer method” is proposed (for example, refer to Japanese Patent Publication No. 2005-93488A1 (corresponding U.S. Pat. No. 7,193,284 (B2)), and “Current-driven excitation of magnetic multilayers”, J. C. Slonczewski, Journal of Magnetism & Magnetic Materials, 159, L1-L7 (1996)). According to the spin transfer method, a spin-polarized current is injected to a ferromagnetic conductor, and the magnetization is switched due to a direct interaction between the spin of conduction electrons constituting the current and the magnetic moment (hereinafter referred to as the “Spin Transfer Magnetization Switching”).
U.S. Pat. No. 6,839,005 discloses a magnetic shift register using the spin transfer. The magnetic shift register stores information by using a domain wall in the magnetic substance. In the magnetic substance divided into a plurality of areas (magnetic domains), an electric current is supplied so as to pass through the domain wall and the domain wall is moved by the electric current. The direction of magnetization in each area is treated as recording data. Such magnetic shift register is, for example, used for recording a large amount of serial data. Meanwhile, the motion of the domain wall in the magnetic substance is reported also in “Real-Space Observation of Current-Driven Domain Wall Motion in Submicron Magnetic Wires”, A. Yamaguchi et al., Physical Review Letters, Vol. 92, pp. 077205-1-4 (2004).
The “domain wall motion MRAM” using the Domain Wall Motion caused by such spin transfer is described in Japanese Patent Publication No. 2005-191032A1 and in “Reduction of Threshold Current Density for Current-Driven Domain Wall Motion using Shape Control”, A. Yamaguchi et al., Japanese Journal of Applied Physics, vol. 45, No. 5A, pp. 3850-3853 (2006).
An MRAM described in Japanese Patent Publication No. 2005-191032A1 includes: a magnetization pinned layer where magnetization is pinned, a tunnel insulating layer stacked on the magnetization pinned layer; and a magnetization recording layer stacked on the tunnel insulating layer. Since including a portion whose direction of the magnetization can be switched and a portion whose direction of the magnetization is not changed substantially, the magnetization recording layer is referred to as not a magnetization free layer but the magnetization recording layer. FIG. 1 is a schematic plane view showing a structure of the magnetization recording layer of Japanese Patent Publication No. 2005-191032A1. In FIG. 1, a magnetization recording layer 100 has a linear shape. The magnetization recording layer 100 includes: a joint portion 103 overlapping with a tunnel insulating layer and a magnetization pinned layer; constriction portions 104 adjacent to both ends of the joint portion 103; and a pair of magnetization pinned areas 101 and 102 formed to be adjacent to the constriction portions 104. The pinned magnetizations opposite each other are applied to the magnetization pinned areas 101 and 102, respectively. Moreover, the MRAM includes a pair of writing terminals 105 and 106 electrically connected to the pair of the magnetization pinned areas 101 and 102. Due to the writing terminals 105 and 106, an electric current penetrating through the joint portion 103, the pair of the constriction portions 104, and the pair of the magnetization pinned areas 101 and 102 of the magnetization recording layer 100 flows.
FIG. 2 shows a structure of a magnetic recording layer 120 of a magnetic memory cell, which is described in “Magnetic Configuration of a New memory Cell Utilizing Domain Wall Motion”, H. Numata et al., Intermag 2006 Digest, HQ-03 (2006). The magnetic recording layer 120 is U-shaped. Specifically, the magnetic recording layer 120 includes a first magnetization pinned area 121, a second magnetization pinned area 122, and a magnetization switching area 123. The magnetization switching area 123 overlaps with the pinned layer 130. The magnetization pinned areas 121 and 122 are formed to be extended in a Y direction, and their directions of magnetization are pinned in the same direction. Meanwhile, the magnetization switching area 123 is formed to be extended in an X direction, and has reversible magnetization. Accordingly, a domain wall is formed on a boundary B1 between the first magnetization pinned area 121 and the magnetization switching area 123 or on a boundary B2 between a second magnetization pinned area 122 and the magnetization switching area 123. Initialization of a magnetization state is carried out by applying a sufficiently-large initial magnetic field in a diagonal direction of 45 degree in an XY plane, and a state where: the magnetization of the magnetization pinned area is directed to a +Y direction; the magnetization of the magnetization switching area is directed to a +X direction; and the domain wall is formed on the boundary B1 is realized after the initial magnetic field is removed.
The magnetization pinned areas 121 and 122 are connected to respective current supply terminals 125 and 126. A writing current can flow in the magnetic recording layer 120 by using the current supply terminals 125 and 126. Depending on the direction of the writing current, the domain wall is moved in the magnetization switching area 123. The magnetization direction of the magnetization switching area 123 can be controlled by the domain wall motion.
However, in the MRAM using the current driven domain wall motion, it is concerned that an absolute value of the writing current becomes relatively large. Other than the above-mentioned Physical Review Letters, Vol. 92, pp. 077205-1-4 (2004), many reports about observation of the current driven domain wall motion are issued. Meanwhile, the domain wall motion requires a threshold current density of approximately 1×108 A/cm2. In this case, for example, in a case where a width of layer causing the domain wall motion is 100 nm and the film thickness is 10 nm, the writing current is 1 mA. To reduce the writing current to be less than this value, it may be considered that the film thickness should be reduced. However, in this case, it is known that the current density required for the writing is further increased (for example, refer to the above-mentioned “Japanese Journal of Applied Physics, vol. 45, No. 5A, pp. 3850-3853 (2006)”). In the MRAM using the current driven domain wall motion, a technique able to reduce the writing current is desired.
Meanwhile, in an element using a perpendicular magnetic anisotropy material whose magnetic anisotropy of a magnetization recording layer is perpendicular to a substrate surface, a threshold current density of around 106 A/cm2 was observed (for example, refer to “Threshold currents to move domain walls in films with perpendicular anisotropy”, D. Ravelosona et al., Applied Physics Letters, Vol. 90, 072508 (2007)).
Relating to the element using the perpendicular magnetic anisotropy material, Japanese Patent Publication No. 2003-110094A1 (corresponding U.S. Pat. No. 6,844,605 (B2)) discloses a magnetic memory using a perpendicular magnetization film and a manufacturing method thereof. The magnetic memory includes: a magnetoresistive element constituted by stacking a first magnetic layer including a perpendicular magnetization film, a nonmagnetic layer and a second magnetic layer including a perpendicular magnetization film, a switching magnetic field of the second magnetic layer being smaller than that of the first magnetic layer, a resistance value of a case where an electric current flows between the first magnetic layer and the second magnetic layer varying depending on a relative angle defined by a magnetization direction of the first magnetization layer and that of the second magnetization layer; and a magnetic field generation mechanism provided to switch the magnetization direction of the first magnetization layer of the magnetic resistance element. The magnetic memory is characterized in that the switching magnetic field Hc of the first magnetic layer expressed in the following expression (1) is set so as to be smaller than the magnetic field generated from the magnetic field generation mechanism.Hc=2(Ku−2πMs2f)/Ms  (1)
Here, Ku and Ms are the perpendicular magnetic anisotropy constant and the saturated magnetization of the first magnetic layer 11, respectively. When the film thickness and width of the first magnetic layer 11 are T and W, respectively, f is expressed in f=7×10−13(T/W)4−2×10−9(T/W)3+3×10−6(T/W)2−0.0019(T/W)+0.9681.
In addition, Japanese Patent Publication No. 2006-73930A1 discloses a method of changing magnetization state of a magnetoresistance effect element using domain wall motion, and a magnetic memory element and a solid magnetic memory using the method. The magnetic memory element includes a first magnetic layer, an intermediate layer, and a second magnetic layer, and information is recorded on the basis of magnetization directions of the first magnetic layer and the second magnetic layer. The magnetic memory element is characterized in that: magnetic domains having mutually-antiparallel magnetization and a domain wall for separating the magnetic domains are constantly formed in at least one of the magnetic layers; and positions of the adjoining magnetic domains are controlled by moving the domain wall in the magnetic layer to carry out information recording. The second magnetic layer may have the magnetic anisotropy in a direction perpendicular to a film surface.
As described above, in the MRAM using the current driven domain wall motion, it is concerned that an absolute value of the writing current becomes relatively large. Accordingly, as described below, the inventors have studied that the writing current can be reduced in the MRAM using the current driven domain wall motion by using the perpendicular magnetic anisotropy material as the magnetization recording layer.
FIGS. 3A and 3B are a plane view and a cross-sectional view of a conceivable magnetoresistive element using a perpendicular magnetic anisotropy material, respectively. A magnetization recording layer 10 includes a magnetization switching area 13 and a pair of magnetization pinned areas 11a and 11b. However, in FIGS. 3A and 3B, a symbol represented by a white circle and a dot, a symbol represented by a white circle and an X-mark, and a white arrow show magnetization directions of the magnetization switching area 13 and the magnetization pinned area 11a and 11b in which they are described.
The magnetization switching area 13 overlaps with a tunnel insulating layer 32 and a pinned layer 30, and has a function as a free layer. The magnetization pinned area 11a is adjacent to one end of the magnetization switching area 13 and the magnetization pinned area 11b is adjacent to the other end of the magnetization switching area 13. Constriction portions 15 are provided to joint portions between the magnetization switching area 13 and the magnetization pinned areas 11a or 11b. Mutually-opposite pinned magnetizations are applied to the pair of the magnetization pinned areas 11a and 11b. In addition, the constriction portions 15 serve as pinned potentials to the domain wall, and the domain wall is initialized so as to be positioned to a region 12a or a region 12b in the vicinity of the constriction portion. The data reading is carried out by using variations of the tunnel resistance depending on the relative directions of magnetizations of the magnetization switching area 13 and the pinned layer 30. Since the magnetization of the magnetization switching area 13 is in a direction perpendicular to the film surface, the magnetization of the pinned layer 30 also has to be perpendicular to the film surface (for example, in Japanese Patent Publication No. 2003-110094A1 and Japanese Patent Publication No. 2006-73930A1).
Meanwhile, regarding the storing and reading of information, it is generally desirable that an MR ratio is large as much as possible. In the past, many reports about a configuration as the magnetic tunnel junction where both of the free layer and the pinned layer have magnetization components in the surfaces are issued. For example, in a case where CoFeB is used for the free layer and the pinned layer and MgO is used for the tunnel barrier, the MR ratio of more than 200% was observed (refer to “230% room-temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions”, D. D. Djayaprawira et al., Applied Physics Letters, Vol. 86, 092502 (2005)). However, in the magnetic tunnel junction where the conceivable magnetic layer having the perpendicular anisotropy is used as shown in FIGS. 3A and 3B, such a large MR ratio has not been observed. Hence, there is a problem whether or not the storing and reading of information can be adequately carried out.